Some large industrial vessels must be cleaned in order to reach desired operational characteristics. Example industrial vessels include large vacuum chambers, autoclaves, pipes, and tubes. For a large vacuum chamber, an internal surface must be cleaned to reach a desired pressure so as to reduce a load on pumps and/or maintain process control of the vessel environment.
Typical residues found on interior vessel surfaces include hydrocarbon contaminants from fingerprints, oil and grease residues from screws and seals, dust and other particulates, and ambient vapors such as water that have been adsorbed into the surfaces. These residues can affect performance of the vessel and must be removed thoroughly.
Typical cleaning measures often involve a time-consuming bake out process for both internal vessel components (e.g., pumps) and a chamber of the vessel. Venting the chamber with dry nitrogen can also prevent more water vapor and other vapors present in ambient air from adsorbing within the chamber walls, but will not generally clean already contaminated surfaces.
Plasma cleaning has also been used to clean interior vessel surfaces, and involves removal of impurities and contaminants from surfaces through use of an energetic plasma created from gaseous species. Gases such as argon and oxygen, as well as gas mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high voltages to ionize a low pressure gas. The plasma formed by passage of the electric current through a low-pressure gas is referred to as glow discharge. Thus, the glow discharge can be created by applying a voltage between two metal electrodes in a chamber containing the gas, and when the voltage exceeds a certain value (referred to as the striking voltage), the gas in the chamber ionizes and becomes a plasma, and begins conducting electricity that causes it to glow with a colored light. The color depends on the gas used.
Existing glow discharge cleaning of interior vessel surfaces typically requires high voltage power sources to sustain the plasma glow discharge created within the large vessel. This type of cleaning is generally inappropriate for very large vessels due to excessive power requirements needed to create the large amounts of plasma. An example cleaning application involves maintaining a diffuse plasma mixture of energetic argon and oxygen ions that bombard the surface(s) of the chamber to be cleaned. But, the high power sources needed to sustain the plasma generation are expensive, hard to come by, and create potential safety hazards as with many high voltage equipment. The high power also requires a steady load on the equipment that will wear out the power supply over time. Thus, glow discharge cleaning may not be practical for large vessels, as the power requirements scale based on an increase in surface area of the vessel.
In addition, the diffuse plasma glow discharge cleaning process is a random process that assumes the paths of the ions will eventually bombard an entire surface needing to be cleaned. However, full coverage for cleaning based on the random paths of the ions would take a long time and is most often unachievable. As a result, typical plasma cleaning methods only support cleaning small objects, and may also not fully clean surfaces having irregular shape.
What is needed is a glow discharge cleaning process that enables surfaces to be cleaned using low power sources for full coverage cleaning.